Focus: The Nanostructure of Buildings and Bridges

X-ray studies reveal the nanocrystalline structure of the essential solid ‘glue’ in concrete.

Concrete is the most widely used substance on earth, besides water, yet the basic structure of its “glue” remains unknown. Now, in the 14 May Physical Review Letters, researchers report on x-ray scattering studies that show that the main binding agent in cement is composed of nanocrystals 3.5 nanometers in size. A better understanding of this structure should aid researchers who are trying to design stronger, more durable, and more environmentally friendly types of cement.

Concrete contains three main ingredients: an aggregate material (such as gravel), cement, and water. Portland cement is the most common cement type, a powdered mixture of calcium, silicate, and various metal ions. When the cement powder is mixed with water and aggregates shortly before use at a construction site, chemical reactions begin that ultimately harden the cement into a solid matrix that holds the concrete together. Calcium-silicate-hydrate (CSH) is the most important binding product from these chemical reactions.

Concrete production operates on a vast scale, and for each ton of Portland cement, about a ton of carbon dioxide is emitted into the atmosphere, accounting for 5 to 7 percent of all human-generated emissions. So researchers have been trying to design processes to make cement stronger and more durable, to ultimately use less of it.

The structure of CSH has not been determined, although previous work has pointed to the presence of a nanometer-scale building block of some kind. To learn more about it, a group lead by Paulo Monteiro from the University of California, Berkeley, carried out x-ray scattering measurements outside Chicago, at Argonne National Laboratory’s Advanced Photon Source, a very intense source of energetic x-rays.

Monteiro and his team directed 115 keV x-rays at samples of “synthetic” CSH, which was carefully created from pure substances in the lab (rather than from the cement industry). X rays scattered off the sample were detected with a two-dimensional detector. The atomic structure of a material leaves its imprint in the interference pattern of the scattered x-rays, with sharp peaks indicating well-ordered crystals.

The interference pattern for the synthetic CSH did not show any sharp peaks, suggesting that there were no large, well-ordered crystals present. But the pattern looked similar to that of the mineral tobermorite, which had been suggested by other researchers as a possible CSH structure.

To compare the synthetic CSH to tobermorite, the team examined the so-called pair distribution function for the scattered x-rays, which gives the probability of finding an atom at a given distance away from another atom in the material. Looking at this function, the CSH peaks corresponding to the shortest distances matched tobermorite’s peaks associated with Si-O and Ca-O bonds. However, there were no observable peaks for distances of more than 3.5 nanometers, indicating that there was no ordered structure larger than that. These results suggest CSH could be made up of densely packed nanocrystals 3.5 nanometers in size, with each well-ordered nanocrystal similar to tobermorite, the team reports. Their results also hint that a slight bending of the Ca-O planes may be responsible for limiting the growth of these crystals to such small sizes.

To prove that the clean signal from the synthetic CSH matched what really happens in concrete, the team also synthesized a “natural” CSH sample, meant to represent the material found in a real concrete structure. From this sample’s x-ray data they subtracted the signals expected from typical impurity crystals and showed that what was left matched the synthetic CSH.

The paper reports “very sophisticated experiments that provide the critical information necessary to refine any model that relates structure to properties,” says Hamlin Jennings, a professor of civil engineering at Northwestern University.

–Michelangelo D'Agostino

Michelangelo D’Agostino is a physicist and freelance science writer in Berkeley, California.